Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Emissions of lean burn engines include particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a Diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as the flow-through monolith carrier, as described herein below) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
One important factor in DOC design is catalyst-deactivation following high temperature exposure. Thermally induced DOC deactivation can occur as a result of sintering of the catalytic component or carrier. Sintering of the catalytic component involves coalescence or crystallite growth of catalytic sites, which are initially well-dispersed. This aggregation results in a loss of surface to volume ratio, reducing catalytic performance. Alternatively, exposure of the DOC to high temperatures can result in sintering of the catalytic carrier. This involves a loss of the carrier pore structure that causes loss of accessibility to catalytic active sites.
As emissions regulations become more stringent, there is a continuing need to develop diesel oxidation catalyst (DOC) systems that provide improved performance, for example, improved NO2 formation at the DOC, which will improve overall performance of the lean burn engine emissions system.